Many processes, such as semiconductor fabrication processes, require water to have an extremely low concentration of ionic and non-ionic impurities. A manufacturing or processing facility with such requirements generally establishes a dedicated water purification plant having suitable treatment capacity and impurity removal characteristics to meet its process requirements. Such a treatment plant typically includes a number of different treatment stages specially selected and arranged to be effective with the particular feed stream received from a local supply, such as a municipal water system, a surface water, a ground water well, treated wastewater or a combination of such sources. When the feed supply is received by the end-user, e.g., by a semiconductor water plant, the in-plant treatment of the resulting feed stream achieves a higher degree of purity by processes such as water filtration, conditioning such as softening or pH adjustment, and deionization, demineralization, degasification or other impurity removal treatments. One common initial treatment is to pass the feed through reverse osmosis (RO) membranes, or through ion exchange beds. High levels of deionization are typically achieved by passing the water through units such as electrodialysis (ED) or electrodeionization (EDI) devices, and distillation may be used in some applications. Organic compounds may pass unaffected through some treatments, or may be introduced or reintroduced by organisms that colonize conduits and tanks in the system. Often these are addressed at one or more stages of treatment steps such as microfiltration, capture in activated carbon or other media, and by breakdown with ultraviolet energy or oxidation. The breakdown or oxidation products may be removed by one or more of the other processes described above.
An in-plant high purity water system may present design problems of various types. The plant must initially be designed to address the range of anticipated feed waters and dependably achieve the minimal required level of water quality. Beyond the factors of capital cost and operating expenses, and the environmental considerations raised by the volume of waste water and by-product of the contaminant-removal treatment processes, various unanticipated problems may arise. Types or levels of contaminants in the feed water may change abruptly, necessitating changes in treatment protocols. The periodically-performed process of regenerating an ion exchange bed, or the unanticipated fouling in the treatment line of a bed or membrane-based treatment system (an RO, ED or EDI unit), may destabilize or impair the treatment process or the quality of the output stream. One therefore seeks to detect problems of this type by the use of diverse monitoring instruments, such as a conductance meters or total organic carbon (TOC) monitors on the output stream or instrumentation elsewhere in the system, and suitable measurements such as off-line ICP-MS measurements. These are applied to develop or maintain robust or effective operating and maintenance procedures. Even so, the continual introduction of new fabrication technologies requires plant operators to frequently ascertain whether existing water quality specs remain sufficient.
Because the normal output water of a high purity treatment plant has such low levels of contaminants, the appearance of an unusual trace contaminant may go undetected when the overall level of the class of contaminants, such as TOC, or other parameter, such as conductance, appears to remain within generally accepted levels. Indeed, an unrecognized or unexpected contaminant might impair the response of the detector, rather than simply passing undetected. In such a case, observation of variation in a process parameter (such as the stability or sensitivity of lithographic exposure or development) or a decrease in quality of a manufactured product (observed, e.g., as an increase in number of defects in a semiconductor wafer) may provide the first indication that the product water has experienced a detrimental change. In this case, investigation is needed to identify the responsible contaminant or treatment unit, and to develop procedures that will, in the future, prevent such quality deviations or detect the responsible agent before it affects the production line. Production down time is quite costly, and the observation of unexplained defects or process variations raises the possibility of additional undetected latent defects, and the specter of defective manufactured products further down the manufacturing train.
Focusing on just one impurity relevant to the present invention, it is generally thought that the presence of boron in UPW product water of a semiconductor fab plant will impair a number of semiconductor processes unless its presence is specifically addressed (for example by effective reduction of the boron load, if necessary, in a first stage, and by use of boron-scavenging resin, ion exchange bottles or other special boron removal unit in a polish loop.) Some fab plants have therefore adopted a conservative approach, removing boron to a very low level, for example by a boron-selective resin column or bed, as shown, for example, in U.S. Pat. No. 5,833,846.
Other ions must also be controlled to below trace concentrations. For this purpose it is common to have a number of exchange resin bottles or tanks in a polish stage of the primary make up water treatment line. Because the ionic concentrations in the final stage are already quite low, the resin can last for an extended period before exhaustion. A conductivity monitor can be positioned after the polisher to provide a prompt indication when the resin approaches exhaustion. When the resin becomes exhausted, ions start to break through, and this condition may be detected by the onset of an increase in conductivity of the product water. A silica detector may also be used to detect the onset of resin breakthrough.
At this stage, it is common practice to send out polish stage ion exchange bottles for regeneration of their resin.
Fab plants typically also have a final polish loop for the UPW water produced by the primary make-up treatment line that has been stored in a tank, to effect final polishing just before the water is pumped out to the various plant processes. Since this final polish loop deals with water that is already substantially deionized, the exchange resin beds or bottles see only small amounts of contaminants and may last for an extended time, e.g., several years, before breakthrough or exhaustion of the resin occurs. These bottles are often replaced with virgin resin, rather than regenerating the resin. Since the simple act of attaching a fresh bottle into the loop, or performing any conduit connections, risks introducing some contaminants into the final loop, it is desirable to carry out such replacements carefully, and as infrequently as possible.
For the polish stages of the primary make up treatment line, the resins are usually regenerated. However, problems may be encountered at this stage. Resin regeneration facilities deal with large quantities of mixed resins from diverse sources. Spent resin from mixed bottles or beds must be separated into anion and cation exchange resins before regeneration, and the separation processes, typically relying on fluidized settling separation properties affected by density, bead size and the like are necessarily imperfect. There is thus a possibility of introducing unanticipated contamination from other resins during various regen resin handling plant operations, e.g., conglomeration, separation by type, regeneration, rinsing, re-mixing and bottle filling. Regeneration of fab plant resins should therefore be performed by a facility that can observe special precautions in the handling of such resins, and the regen process should be tightly controlled or specified. Often, plants will have only one qualified vendor. Larger fab plants may perform their own regeneration, while some fab plants may simply require that exhausted beds be replaced with entirely new, rather than regenerated, resin.
Boron is a weakly bound ion. In operation, ions captured by ion exchange from product water in an exchange bed bind to the exchange resin, and weak ions may be displaced by other ions having a stronger affinity for the resin. The more weakly held ions are therefore continuously displaced and shifted toward the downstream end of the ion exchange bed as the upstream end becomes more saturated. The more weakly dissociated species are also captured with lower efficiency, and may extend diffusely along a relatively long depth of the ion exchange bed. Boron, in particular, has a non-self-sharpening wave front and moves through the bed well ahead of other ions. Silica, a common and weakly held ion, has recently been regarded as a good breakthrough indicator of bed exhaustion, and it may be easily detected, for example by a colorimetric, wet chemistry silica detector. The above-noted conductivity rise has also generally been considered an effective indicator of impending breakthrough, and can be detected by a common resistance meter placed downstream of the polisher.
It should also be noted that some fab plants have specified a zero detectable boron standard for their process water. This has lead to the presence of boron being addressed by various approaches, such as the replacement of the polish bed whenever boron concentration reached the detection threshold. One group has reported, however, that the latter method resulted in the need for extremely frequent regeneration of the polish bed—over one hundred times per year. They proposed instead an approach of using of a boron-selective capture resin at various places in the treatment stream ahead of the polish bed to reduce the boron load on that unit.
Recently, it has been noted that a boron breakthrough may be detected earlier than the silicon breakthrough, and before a detectable conductivity rise. For this purpose, boron concentration is monitored directly, using a sufficiently sensitive boron detection instrument. The appearance of boron in treated fab product water may then be used as an indicator of impending exhaustion of the polishing bed exchange resin. Boron is displaced earlier, preceding the breakthrough of silica, and as such constitutes an indicator that may allow a more accurate determination of, or at least an earlier, hence more secure anticipation of, the exhaustion of a normally-functioning ion exchange bed. For such specialized boron detection, one instrument maker (Sievers Instruments, Inc. of Boulder, Colo.) has developed a very sensitive boron detection instrument for UPW monitoring and treatment process control. That boron monitor, which is described in published International Patent Application WO 02/12129, now permits the detection of boron concentration at very low levels, e.g., at parts-per-trillion (ppt) concentration levels. That international patent application is hereby incorporated herein by reference in its entirety.
Boron concentration measurements made with such a detector may in principle be used to anticipate bed exhaustion and to determine timely maintenance, such as replacement or regeneration, of ion exchange beds, thus avoiding unanticipated deterioration of product water quality or costly shut-down of the water production. The detection should permit one to schedule bed replacement or regeneration well before the occurrence of leakage of silica, or of other more tightly bound and more destructive ions, through the polish unit in the product water.
However, boron is a loosely bound ion. If one is controlling based upon a very low detection threshold, it is important that the level or shape of the boron concentration curve be discerned, distinct from background. As noted above, it is also generally accepted that the absolute level of boron should be relatively low. However, early measurements with a sensitive detector have uncovered great variations in the boron-passing or release characteristics in a UPW product water polish stage. Boron is loosely held, is easily displaced, and is captured with fairly low efficiency by the remaining downstream resin. An aged resin bottle, which has already accumulated a load of boron ions, will release boron ions in proportion to the total ionic load as it nears exhaustion, increase the residual level of boron when the water temperature rises a few degrees. More significantly, applicants have observed that sometimes newly-regenerated resins appear to release a large amount of boron. It appears therefore that at least some regeneration processes do not produce regenerated resin capable of sustained and dependable boron removal.
It would therefore be desirable to provide a regenerated resin having lower passing or release characteristics, and/or more effective and long-lasting capture characteristics
It would also be desirable to provide an improved resin regeneration process that dependably produces regenerated resin having lower boron passing or release characteristics, or more effective and long-lasting boron capture characteristics.
It would also be desirable to provide an improved process for producing low-boron UPW product water.